Here's a nice way to make a "factory looking" cold air intake for your E12. It uses the stock air cleaner box and other BMW parts. The whole thing costs around $12.00. I just put one on my 81 528i.
You need to get one Suction Tube (13-71-1-287-326) $6.00. This attaches through the hole you need to cut in the headlight cover. Center the 3.16" hole in the lefthand section of the cover. I used a jigsaw to cut the hole. You also need one Tube/A-Cleaner (13-71-1-287-327) $5.73. This attaches between the air box and the Suction Tube. I cut 1.8" from the air box inlet, but you may be able to collapse the Tube/A-Cleaner enough that you won't need to. Measure twice cut once.
These parts were used on the e24 (635i), and maybe others for many years. The installation looks factory and may add a ram-air effect at speed.
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